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Aviation History
1959
1959 - 0785.PDF
384 FLIGHT, 20 March 1959 AERO ENGINES 1959 . . . investigated five machines: fixed wing, four turboprops, largeflaps, deflected slipstream; four turboprops, tilting wing; fixed wing with tip-mounted, tilting ducted propellers; standard twinturboprop with fuselage-mounted jet lilt; and an advanced con- figuration with lift/propulsive turbojets distributed along the rearportion of the wing, which could be hinged downwards. Many of the results obtained from the thousands of calculationsperformed were astonishing, and serve to emphasize the folly of jumping to conclusions; for example, the fifth (all-jet) studyproved to be the cheapest to operate on either a single- or a split- level mission. However, the basic conclusion of the study wasthat there was "real promise of satisfying the requirement" only with the first two configurations, and that of these the second(tilting wing) was superior. Turning to considerations of actual hardware, Breguet have chosen the first layout (but have evolveda sophisticated and advanced vehicle), while Hiller have, to U.S. Navy contract, adapted a once-conventional logistic transport tothe second—the optimum tilt-wing—configuration. When the Hiller X-18 starts flying its behaviour should be worthy of study. From the engine viewpoint the conclusions drawn are veryacceptable. All that is needed is further development of turbo- props of maximum power/weight ratio, together with large-dia-meter propellers of minimum weight and the associated reduction gearing and wing-tilting mechanisms. Nothing very arduous; yetit is in meeting just such straightforward requirements that an individual firm can beat its competitors. To an ever-increasingdegree, success will go to companies whose basic approach is right, and will elude any company which fails to appreciate that,for example, the best engine for a tilt-wing machine may have to be developed from a clean sheet of paper.There are, of course, many other types of aircraft in which direct lift is provided. One family which may well become ofmajor importance is that which embraces the profusion of VTOL tactical combat aircraft which promise to spawn by the thousand.Discounting propeller-driven machines, such as the American XFV-1 and XFY-1, the first of this family actually to be builtis the SNECMA C.450 Coleoptere. As is well known, this consists chiefly of a single turbojet around which is mounted an annularwing. After taking off from a tail-sitting attitude the C.450 is controlled during engine-supported flight by aerodynamic jet-deflection accomplished by injecting compressor-bleed air from selected inwards-facing slits around the engine propulsive nozzle.After transition to wing-supported flight, control is accomplished by aerodynamic surfaces carried around the wing.There is much to be said for such a system, which appears in every way preferable to pivoting the powerplant. (The converseis true of large rocket motors, as is presently noted.) In a recent public lecture Dr. S. G. Hooker, of Bristol Siddeley Engines,confirmed his company's interest in such arrangements, saying [colloquially], "Our approach is to employ swivelling jets, not ofthe engine itself but the actual jet thrust flow." It has several times been suggested unofficially that an arrangement of thiskind is to be the key to the Hawker P.I 127, the third-generation attack aeroplane for NATO. Officially described merely as havinga "special Bristol engine," it seems fairly clear that the P. 1127 will be the first fixed-wing fighter capable of rising off the groundvertically in a normal flying attitude (it may be preceded by the Bell XF-109, for which a U.S.A.F. contract has already been let). There are several methods by which the thrust axis could bevaried to suit the different regimes of flight. All such systems arc A suggested lifting / propulsion powerplant for a relatively high-speed fixed-wing VTOL aircraft by American G.E. Some comments on this arrangement are given at the top of column 2 UFTflM likely to be designed for optimum efficiency in the translationalregime (either at cruising or at supersonic maximum speed) and so specific consumption in the hovering mode will be high—probably well over 1.5. Such an arrangement would therefore be unattractive for anyvehicle intended for prolonged hovering. It is, however, quite possible to invent propulsion systems with even more variedcharacteristics, and a number are discussed in the paper Con- vertible Turbojet Engines for VTOL Aircraft by Messrs. Zipkin,Rossbacn and Brown of American G.E. and read earlier this year. A typical such engine is shown below. Amongst theadvantages claimed for such a powerplant are improved matching between cruise and lift thrust, thrust vectoring without engine orwing rotation, no limitations of flight speed, the considerable growth-potential consequent upon the use of interburning (com-bustion of additional fuel between the gas generator and the tip turbine) and the fact that existing gas generators may be used.To meet the engine-out case it is suggested that four such power- plants should be interconnected by cross-ducting. Among a range of applications suggested, the authors of thispaper cite the case of a projected observation aircraft, with a gross weight of 13,500 lb including a crew of two and 600 lb ofelectronics. Powered by two J85 engines, with a single fuselage- mounted lift fan, the aircraft could take off vertically, fly 200 n.m.at sea level, loiter for two hours, cruise back and land vertically; with zero loiter time the cruising range would be 700 n.m. Themaximum speed would be 550 kt (M 0.83). This work follows upon earlier studies by G.E. which were summarized in thisjournal on February 1, 1957. Ballistic Vehicles It is appropriate to say a few words about the probable futurepropulsion of large ballistic missiles, and orbital or skip vehicles which are not intended to leave the neighbourhood of the earth.At present there seems little doubt that for several years the most important propulsion systems in this field will continue to belarge liquid-propellant rocket motors, using liquid oxygen as the oxidant. It is safe to assert that such powerplants will provideat least 95 per cent of the total impulse required. Rocketdyne's method of chamber construction—the basket-tube system, des-cribed in the section dealing with that company in the catalogue section of this issue—is likely to be widely adopted. It eliminatesmany manufacturing problems, facilitates the construction of chambers with Prandtl nozzles (an optimum divergent nozzlewith a concave interior form) and results in an assembly of mini- mum weight. Vehicle control will invariably be accomplished bygimballing the chamber rather than by deflecting the jet with refractory vanes. There is, of course, tremendous scope for development in theclassic liquid-propellant-engine. Not only can units be made progressively smaller and lighter for a given rating but, providedimmense financial and technical support can be made available, engines can be produced with much higher thrust than any yetattempted. As an initial, rather crude step, Rocketdyne are developing a cluster unit consisting of an octet of Thor/Jupiter-size chambers, uprated to give an aggregate thrust of 1,500,000 lb. This could place 10,000 lb in a stationary Earth orbit (22,400-mileradius) or land 2,000 lb gently on the Moon. Rocketdyne ako hold a U.S.A.F. contract for a next-generation engine, with a singlechamber rated at 1,000,000 lb growing to 1,500,000. In a recent paper, T. F. Dixon, Rocketdyne's chief engineer,pointed out that development of such a huge unit will be a formid- able task; "turbomachinery capable of the power outputs of smallmunicipalities will be required to feed the propellants to the chamber, stable combustion chambers will have to be built withcombustion densities many times that of existing engines, and plumbing, valves, and so forth, will be beyond the range of currentexperience." Nevertheless, giant rockets will be essential; and, to show that Rocketdyne look ahead, Mr. Dixon goes on to pointout that a 20 X 106 lb engine could place 300,000 lb in a low Earth orbit, take 200,000 lb and land it on the Moon or orbit 60,000 lbround Mars and return it to the Earth. Notwithstanding the great strides which continue to be made inthe field of solid propellants—especially in the military field, where they are particularly attractive—it does not appear likelythat they can be employed in high-performance vehicles where the achievement of maximum specific impulse* is all-important. Testfirings are already taking place with fluorine, in conjunction with derivatives of amine (such as UDMH); other reactive oxidizerscan achieve specific impulses theoretically higher than those attainable by any solids. Ultimately the highest chemical specificimpulse should be achieved by engines running on liquid hydrogen and either liquid oxygen, liquid fluorine or liquid ozone. On p. 385 it can be seen how low is the best specific impulsewhich can be achieved by any chemical rocket. When all the possible combinations are exhausted it will still be possible totake a major jump by discarding today's molecular propellants *Specific impulse I is thrust obtained from a given rate of propellant consumption; if the units are Ib and sec, I is measured in sec.
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